The present invention relates to ceramic compositions of high density and high hardness, and more particularly to sinterable compositions for ceramics based on zirconia, alumina, spinel, mullite or cordierite, which compositions exhibit the capability of being sintered to highly dense products exhibiting no or low open porosity at greatly reduced sintering temperatures.
Ceramic materials exhibiting high hardness and high density are useful for a variety of applications including, for example, armor materials, wear materials, materials for cutting edges, precision machine ceramic components, and other applications. It is known that, when ceramics of these types do not have to withstand particularly high temperatures, sintering aides can be used to reduce the difficulty of fabrication. Hence, when added to the base ceramics these aides lower the temperatures to which the ceramic precursor powders must be heated to sinter them to full density with low open porosity.
I. B. Cutler et al., in "Sintering Alumina at Temperatures of 1400.degree. C. and Below", J. Am. Ceram. Soc., 40 (4), pages 134-139 (1957), describe alumina ceramics containing manganese oxide, copper oxide, titanium oxide, and combinations of titania with manganese oxide or titania with copper oxide wherein sintering temperatures as low as 1300.degree. C. could be achieved. W. R. Cannon, in "High Creep Ductility in Alumina Containing Compensating Additives", Advances in Ceramics, Vol. 10, Structure and Properties of MgO and Al.sub.2 O.sub.3, pages 741-749, Am. Ceram. Soc., (1984), added 2 mol percent of each of copper oxide and titania to achieve alumina sintering at 1200.degree. C.
U.S. Pat. No. 4,719,188 discloses abrasion-resistant aluminas comprising up to 4% total of TiO.sub.2 plus CuO in combination with 0.5-4% of various other transition metal oxides including manganese oxide. These materials were reported to be sinterable at 1200.degree. C.
Sinterable zirconia compositions have also received considerable attention. Hence M. Kimura, in "Preparation of Low-Y.sub.2 O.sub.3 -TZP By Low Temperature Sintering", Science and Technology of Zirconia, III, pages 183-191, Am. Ceram. Soc. (1988), describes powdered yttria-stabilized zirconia containing copper or manganese oxide which may be sintered at temperatures as low as 1200.degree. C. See also I-Wei-Chen, U.S. AFOSR Report No. AD-A200-202 (Aug. 25, 1988).
C-M. J. Hwang et al., in "Effect of A Liquid Phase on Super Plasticity of 2% Mol-Y.sub.2 O.sub.3 -Stabilized Tetragonal Zirconia Polycrystals" J. Am. Ceram. Soc., 73 (6), pages 1626-1632 (1990), achieve the sintering of colloidal stabilized zirconia containing small additions of copper oxides. Also, I-Wei Chen et al., in "Development of Super-Plastic Structural Ceramics", J. Am. Ceram. Soc., 73 (9), pages 2585-2609 (1990), describe super plasticity in stabilized zirconia polycrystalline products comprising copper oxide additions.
While the foregoing publications indicate considerable progress in reducing the sintering temperatures of hard ceramics such as alumina and zirconia, it will be evident that still further reductions in such sintering temperatures would be useful. Of particular interest for many of the above-noted applications for hard ceramics would be sinterable ceramic compositions achieving low porosity and high density at sintering temperatures in the 800.degree.-1100.degree. C. range. To date, no combination of sintering aides effective to reproducibly provide dependable sintering at these temperature, while maintaining high density and hardness in the product, has been discovered.